i bumped up to 2 gigs. it makes a huge difference. you'll notice a little when you go an extra 512. i reccomend going another gig. 2 512 sticks of ram. depending on the chipset of your computer, there is a feature called "dual channel" where the 2 sticks split the read and write process. you'll notice a big difference with that. plus ram is cheap as all hell right now. buy while its hot.
I upgraded to 1.24 gig from 256 mb before. I can definitly tell a difference. Mainly when working with music. It tends to be alot more crisp. But, I think I am going to go cop a new processor as well..
I believe if you use DDR RAM you have to keep the same sizes in both slots in order to receive the data rate benefit. So I'd recommend either two 512s or 2 Gigs.